When you're gonna do multiple exposures, you have to take into account the total amount of light needed to make a "proper exposure."
For example, say your meter tells you that a proper exposure for a certain scene is 1/500 at f/8. The total exposure for the frame shouldn't exceed this, or your photo will be overexposed. so for a double exposure, you'd need to set your camera to 1/1000 at f/8 and shoot twice.
It helps to think of a frame of film as a bucket and light as water. Each frame will only "hold" so much light. This is why it is SO important to have a good grasp of how film speed, f/stop and shutterspeed interrelate to make an exposure.
/edit to recommend a book by Bryan Peterson called Understanding Exposure. He does a great job of explaining the whole concept of the realtionship of shutterspeed, f/stop and film speed. His approach is very readable and easy to understand.